It's amazing that people can just come out and say "I know what the God meant, while people who lived this tradition for thousands of years, carried it over several continents and kept it alive through wars, plagues, genocides, raise and ruin of empires - they all don't know what they're talking about". It certainly is gutsy.

The thing is, there's only two ways to know things, in general. One is experience it yourself - in this case, you go to God, ask Him (or Her, or Them, whatever works for you) what is the meaning of that, get an answer and live the rest of your life according to it (or ignore it if you like, your choice, free will and stuff). Another is trust the relevant authorities - and in this case, those are the rabbis. If you are interested in Judaic tradition, then there's no Judaic tradition outside the rabbis. Of course, inside there's a lot of variety, but if you discard the rabbis, then you've got no Judaism. Whatever is left is not based on Judaic tradition anymore, and then one would have to ask - what exactly is it based on? Why would you believe anything in those texts at all or put any importance into it? Of course, you don't have to - but then the question of what's the right thing to do on Shabbat is moot - you do whatever you like (well, excluding the obvious things like murdering people etc.) and it's fine.